A converter station in an installation for transmission of high-voltage direct current is connected between a three-phase alternating-voltage network and a dc connection and comprises a converter, a transformer coupling for connection of the converter to the alternating-voltage network and shunt filters for generating reactive power and filtering harmonics. The transformer coupling may consist of one or more physical units. The converter is normally a line-commutated current-source converter, by which is to be understood that the commutation between the valves of the converter takes place by means of voltages occurring in the alternating-voltage network and that the dc connection, viewed from the converter, occurs as a stiff current source. For the purpose of reducing the harmonics generated by the converter, especially the 5th and 7th harmonics, the converter is designed as two 6-pulse bridges which are series-connected on the direct-voltage side and which are each connected to a respective three-phase alternating-voltage system with a mutual phase shift of 30.degree.. This mutual phase shift may be achieved by arranging the transformer coupling to comprise two three-phase secondary windings, one being connected in a star connection (Y connection) and the other in a delta connection (D connection). The transformer coupling further comprises a usually Y-connected primary winding with a grounded neutral point. For a general description of the technique for transmission of high-voltage direct current, see, for example, Erich Uhlman: Power Transmission by Direct Current, Springer-Verlag, Berlin, Heidelberg, New York 1975. In particular FIG. 2.7 on page 15 shows the configuration described above.
It is known to series-compensate converter stations by connecting series capacitors into the connection leads between the alternating-voltage network and the ac connections of the converter bridges. This results in several advantages. The series capacitors are charged periodically by the current flowing through them and the voltage thus generated across the capacitors gives an addition to the commutating voltage across the valves of the capacitors. The commutating voltage becomes phase-shifted relative to the voltages of the alternating-voltage network in such a way that, with control and extinction angles (margin of commutation) related to the phase position for the voltages of the alternating-voltage network, the valves may be controlled in rectifier operation with control angles smaller than zero and in inverter operation with extinction angles smaller than zero. This makes possible a reduction of the reactive power consumption of the converters and also provides a possibility of generation of reactive power. This reduces the need of generation of reactive power in the shunt filters and these may thus be dimensioned substantially based on the need of harmonic filtering. The charging current of the capacitors and hence the voltage thereof are proportional to the direct current in the dc connection, and by a suitable dimensioning of the capacitors the dependence of the overlap angle on the magnitude of the direct current may be compensated. This means that the series compensation contributes to maintain the margin of commutation even in case of rapid current transients. Also the dependence of the margin of commutation on the amplitude of the alternating-voltage network is tarorably influenced through the series capacitors.
Thus, in many contexts it is desirable to series compensated converter stations of the kind described above. In the known solutions which have been proposed so far, the series capacitors have been placed between the respective secondary windings of the transformer coupling and the ac connections of the converter bridges, which means that six capacitor units must be installed. By instead placing the capacitor units between the primary winding of the transformer coupling and the alternating-voltage network, the number of capacitor units may be reduced to three. However, it has been found that, with this arrangement and, for example, in case of a ground fault in the alternating-voltage network, the capacitor units are subjected to impermissibly high voltages, in particular since the overcurrents occurring in case of such ground faults may be amplified by resonance between the alternating-voltage network including the shunt filter and the capacitor units and the transformer coupling. Even if the series capacitors are normally equipped with overvoltage protection means, for example in the form of surge arresters, the stress thereon in the form of developed energy becomes unrealistically high since faults of the above-mentioned kind often remain for a relatively long period of time.